1. Field of the Invention
The present invention is related to a production method of a thin film resistance element formed on or buried in a printed circuit board, and the thin film resistance element employing the same method, in particular, the production method capable of precisely controlling a thickness and a shape of the thin film resistance element.
2. Description of the Related Arts
Generally, a printed circuited board used in various kinds of electric facilities is further demanded to be reduced in size and weight, in particular, how to install electrically resistance bodies into the printed circuit board is a key not only to reduce the size and weight of the printed circuit board but also to maintain an accuracy value of resistivity of the resistance bodies. As one of examples for forming these resistance bodies in the prior arts, there is one where a printed circuit pattern is formed on, for instance, a ceramic substrate (core) and the resistance bodies are formed by applying an electrical resistance paste on the printed circuit pattern by using a printing method. This printing method has been widely used ever since, and the resistance bodies formed on the printed circuit pattern is referred to as a printed resistance body.
FIGS. 10(A) and 10(B) are sectional views for explaining a production method of a resistance body by a printing method in the prior arts.
Next, a description is given of the production method of the printed resistance body in the prior arts, referring to FIGS. 10(A) and 10(B).
In FIGS. 10(A) and 10(B), a character 2 designates a ceramic substrate (core) with an insulating layer on the surface thereof or maintaining an electrically insulating state. As shown in FIG. 10(A), on the surface of the ceramic substrate 2, an electrically conductive paste of Agxe2x80x94Pd is applied by, for instance, a screen printing method, resulting in a pair of electrically conductive pads (referred to as conductive pads) 4, 4 separated at a certain distance to each other. Then, on the surface of the ceramic substrate 2, an electrical resistance paste is applied by the screen printing method, resulting a printed resistance body 6 between the pair of conductive pads 4, 4 separated to each other at a predetermined distance L as shown in FIG. 10(B).
The value of resistivity of the printed resistance body 6 depends on the dimensions of the printed resistance body 6, i.e., a resistance length L, a resistance width W (not shown) and a thickness t of the printed resistance body 6. Since the value of resistivity of the printed resistance body 6 is varied according to the dimensions, there arise problems as follows.
First, upon applying the electrically conductive paste and the resistance paste on the ceramic substrate 2 by the screen printing method, a shift of printing and penetration of these pastes inevitably occur, resulting in a deviation of value of resistivity in the printed resistance body 6.
In particular, a thickness of the resistance paste printed on the ceramic substrate 2 varies largely because of a difficulty to control printing conditions such as a squeezing pressure, a squeezing angle and a viscosity of resistance paste, resulting in an increase of the deviation of value of resistivity in the printed resistance body 6.
The conductive pads 4 made of Cu (copper) give poor ohmic contact. This causes a generation of an excess value of resistivity at connecting portions of the conductive pads 4. This fact poses a difficulty to obtain a designed value of resistivity with the printed resistance body 6.
Generally speaking, the deviation of value of resistivity thereby is as large as about xc2x130%. This fact implies to require an additional adjustment process such as trimming for correcting the value of resistivity to a designed value at the final stage.
Accordingly, it is a general object of the present invention to provide a production method of a thin film resistance element formed on a printed circuit board (core material), and the thin film resistance element produced by the same method, in which above disadvantages have been effectively eliminated.
More specific object of the present invention is to provide a production method of a thin film resistance element formed on a printed circuit board, and the thin film resistance element produced by the same method, wherein the thin film resistance element can be formed in such a manner that the dimensions such as the thickness thereof are controlled in high accuracy.
Another specific object of the present invention is to provide a production method of a thin film resistance element formed on a printed circuit board, comprising the steps of forming a thin film resistance layer having a predetermined thickness on the printed circuit board through an insulation layer by a dry process used in producing a semiconductor; forming an electrically conductive layer on the thin resistance layer, and etching the electrically conductive layer selectively so as to make, at least, a pair of electrically conductive pads, resulting in the thin film resistance element having a predetermined value of resistivity between the pair of electrically conductive pads.
Another more specific object of the present invention is to provide a thin film resistance element formed on a printed circuit board having a circuit pattern on a substrate and an insulation layer covering the circuit pattern having a thin film resistance element formed on the insulation layer of the printed circuit board by a dry process used in fabricating semiconductors; and at least, a pair of electrically conductive pads formed on the thin film resistance layer, the pair of electrically conductive pads being separated at a predetermined distance to form a thin film resistance element between the pair of electrically conductive pads.